Telecommunications network

ABSTRACT

In a telecommunications switching network which has a plurality of sub-networks, each sub-network having a plurality of User Network Ports, the Destination and Source addresses in a cell-header carrier by a cell switched by the network are each divided into a Port Address part and a Sub-Network Address part, the Source Sub-Network part being implicit while the cell remains in the Source Sub-Network and similarly the Destination Sub-Network part is implicit when the cell arrives in the Destination Sub-Network, the Source and the Destination Sub-Networks being linked via a Cross-connect switching device wherein the address formats are changed.

BACKGROUND OF THE INVENTION

In United Kingdom Patent Application No. GB 2255257 is described anearlier concept for an Asynchronous Transfer Mode (ATM) self-routingnetwork which may be used for Private Networks, Virtual Private Networksand distributed Local Area Switches in the Public Network. The serviceis connectionless using a, special format of the address octets i n thecell header; cells are directed to their respective destinations by anInterpreter held in each elemental switch or at each switch input port,depending upon implementation. No overhead is required in the cellInformation Field which therefore corresponds to the payload in the ATMlayer or a composite network; hence the use of standard adaption-layerformats is not compromised. This application is included herein byreference.

The network is divided into Mini-Networks typically with up to 256 userNetwork Ports. The size of the address field limits the number ofMini-Networks served to 16. The resultant cell header format is shown inFIG. 1.

It will be seen that such a network is limited to a maximum of 4k(4,096) User Network Ports, although thee use of multiplexers cansubstantially increase this number.

SUMMARY OF THE INVENTION

According to the present invention there is provided atelecommunications switching network comprising a plurality ofsub-networks, each sub-network having a plurality of User Network Portswherein the Destination and Source addresses in a cell-header carried bya cell switched by the network are each divided into a Port Address partand a Sub-network Address part and wherein the Source Sub-network partis implicit whilst the cell remains in the Source Sub-network andsimilarly the Destination Sub-network part is implicit from when thecell arrives in the Destination Sub-network, wherein the Source and theDestination Sub-networks are linked via a cross-connect switching devicewherein the address formats are changed.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example, withreference to the accompanying drawings, in which:

FIG. 1 shows an example of a prior art cell header format;

FIG. 2 shows a variation of the format shown in FIG. 1 with thecross-connect function of the present invention;

FIG. 3 shows a further variation of the format shown in FIG. 1 having amodified cross-connect function;

FIG. 4 shows diagrammatically the paths between users on twomini-networks.

FIGS. 5 and 6 show diagrammatically the transmit and receive functionsfor a public network link;

FIG. 7 shows diagrammatically the signalling system between peripheralnodes for the present invention;

FIG. 8 shows diagrammatically the use of a Public Network Port addresswith addresses allocated in site networks.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The earlier network, as described above, uses a common address formatthroughout (apart from changes at the Public Network Port used inmapping to Public Virtual Circuits (PVCs) for inter-site links). Thismeans that the Source Mini-Network address is carried in the sourceMini-Network where it is redundant, being implicit, and similarly theDestination Mini-Network address is carried in the destinationMini-Network. FIG. 2 shows how, by eliminating this redundancy, inaccordance with the present invention, the address capability may beexpanded.

In the figures shaded areas show where changes have been made from theearlier format.

Two slightly different formats are used for the Source and DestinationMini-Networks respectively and, since a Mini-Network is relativelyunstructured internally, it is necessary to inform the interpreterswhich format is being used and this is done by using the leastsignificant bit of the most significant octet of the cell header as aSource/Destination (S/D) format indicator. Within the cross-connectfunction both Source and Destination Mini-Network identities must beknown but, since the total information required to be held exceeds thecapacity of the standard cell header format, the function must beinternal to a switch element.

The rules for Interpretation are as follows:

If the S/D bit is zero AND the D-Minet 1,2 field indicates ownMini-Network then route on D-Port 1,2.

If the S/D bit is zero AND the D-Minet 1,2 field indicates that thedestination is NOT own Mini-Network then route on D-Minet 1,2.

If the S/D bit is one then route on D-Port 1,2.

The theoretical maximum addressing capability using the cell headerformats of the present invention is 128 Mini-Networks with 256 UserNetwork Ports (UNPs) each. The resultant maximum address range is 32kUNPs which is a very large number for a private broadband network.

In order to improve fault tolerance and to simplify congestion avoidancehalf of this capacity is sacrificed by pairing Mini-Networks so that twoadjacent addresses serve the same group of UNPs. The LS bit of the Minerfield is used to separate the two, this bit is referred to as the `A`(for Alternative route) bit.

The A-bit is still included in the interpretation of the Minet field andis also included in interpretation of the Port field. By this meansalternate paths are provided both for intra-Mini-Network and tocross-connect switches and public network ports.

The rules for interpretation are therefore modified as follows:

If the S/D bit is Zero AND the D-Minet 1,2 field indicates ownMini-Network then route on D-Port t,2 and the A-bit.

If the S/D bit is zero AND the D-Minet 12 field indicates that thedestination is NOT own Mini-Network then route on D-Minet1,2 and theA-bit.

If the S/D bit is one then route on D-Port 1,2 and the A-bit.

It will be seen that the A-bit is included in the interpretationwhatever the state.

The value of the A-bit is normally not the user's concern, being set inthe Exchange Terminator (ET) from a table for the 16 possible VirtualConnections (VCs) and maintained by the Resource Manager. However, forcertain functions (for example Multicast codes), the two addresses maybe used individually. The S/D bit, normally used to identify the headerformat, is redundant in the upstream UNP link; and is used instead toinform the ET that the A-bit value given is real. Note that this featureis implemented by user choice, in a network where the address range isnot a pressing problem users may prefer to leave the choice up to themanager.

If the connection is to another Mini-Network the A-bit (in fact theDestination A-bit (DA-bit)) would be used to select one of two separatecross-connect switches. In the chosen cross-connect switch the DA-bit isignored. The cross-connect function will replace the D-Miner field withthe S-Minet field and set the S/D-bit to `1`. The Source A-bit (SA-bit)will be set from the interpreter to a value chosen by the ResourceManager resulting in two routes to the destination UNP from each of thetwo cross-connect switches. It will be seen that, by this means, thechoice of preferred path is localized in each Mini-Network. FIG. 3 showsthe modified connect function.

The DA-bit may also be used to choose one of two Public Network Ports;however this bit is ignored in the transmit function it being assumedthat the public network provides its own means of avoiding congestionand faulty equipment. At the Public Network Port receive function theSA-bit is added in thee same way as for a cross-connect.

FIG. 4 illustrates the alternative paths available from a user on UNP"X" on Mini-Network "P" to a user on UNP "Y" on Mini-Network "Q" andalso to a user on his own Mini-Network and to the Public Network ports.

The theoretical maximum addressing capability using the cell headerformat of the present invention is 128 Mini-Networks with 256 UserNetwork Ports each; however, in a Virtual Private Network (VPN) usingPVCs on ATM cross-connects in the Public Network to interconnect sites,the maximum is unlikely to be achievable. The reason for this is thatthe Public Network is performing the cross-connect function describedearlier but, being a large and distributed network, it cannot offer theincreased address space available internal to a cross-connect switch.This problem is reduced, but not eliminated, by the alternative-routingfeature (A-bit) which halves the effective number of Mini-Networksserved over public network links and is further reduced by using theA-bit real address feature, where two Public Network Ports are fittedwhich may, in this case, be accessed under user control and havedifferent translators.

The User-Network Interface (UNI) format for links with the publicnetwork is too restrictive with the VPI field limited to 8 bits;however, it is possible to negotiate an Network-Network Interface (NNI)access format with the public network operator. The Generic Flow Control(GFC) field is lost but public network generic flow control adds littleto the inbuilt congestion control of the VPN management tools, whichmonitor queue lengths and provide a service-friendly mechanism forrestricting bandwidth on specific links. All that is necessary inaddition is a communication channel between the private network managerand the public network manager, a channel provided in the site networksby the network signalling.

Even using the NNI format on the UNI link the address capacity is stillnot enough for a single access link to address the complete network. Inthe scenario described below User Network Ports (UNPs) are of two types:

Those with access to/from the Site network only (type "S).

Those with Universal access (type "U").

Unlike the private network described earlier where the Virtual PathIdentifier (VPI) and Virtual Channel identifier (VCI) cell header fieldscarry absolute addresses for UNPs in the private network, these fieldsin the public network provide a unique path/channel identity only on asingle link. Excluding the LS 4 bits of the VPI field there are 24 bitsavailable and the value carried in these bits must uniquely identify, atthe source end:

The identity of the (type U) destination UNP in the total . externalnetwork.

The identity of the (type U) source UNP on the source site.

Rule 1

The product of the total number of User Network Ports of type U servedby the link in the external network, and the number of User NetworkPorts of type U served by the link in the source network, must notexceed 2²⁴.

ATM switches used in a public connection-mode network have to providetranslation of the address field values. For ATM transport networkcross-connect switches it is likely that translation is limited to the12-bit VPI field thus:

Rule 2

The number of destination sites served by the link must not exceed 2¹².

The VCI field content is carried unchanged by the public network.

A complete network address is 15-bits long. Full source plus destinationaddresses would require 30 bits, but the ATM cell header (less the4-bits used for the MUX field) provides only 24 bits; this availableaddress space must be used as efficiently as possible. The alternativerouting feature reduces the Network Port address to 14-bits so that theoverlap between Source plus Destination addresses and the availableaddress space on a public network PVC from 6-bits to 4-bits.

The space occupied by the source address depends upon the number of typeU ports to be served; similarly the space required for the destinationaddress depends upon the number of type U ports in the rest of thenetwork. The boundary between these two number domains is variable,depending upon the configuration of the network; the maximum size of thesource and the destination domains is 14 bits (assuming alternativerouting is implemented) which implies a boundary region of 4 bits.Separate translators for source and destination addresses are required;each translator gives a 14-bit output word and the two words are mergedwith a 4-bit overlap. In the overlap region the two words are added; theeffect of this is that the boundaries between the two domains need notbe on a power-of-two boundary although in most cases, for ease ofmanagement, it will. The resultant interface function is shown in FIG.5.

The corresponding function at the receive end of the link is shown inFIG. 6.

Note that the transmit function is closely associated with a multiplexerfor inputs from several site Mini-Networks; similarly the receivefunction is associated with a demultiplexer for outputs to several siteMini-Networks.

The value of broadband ports which only have access to the on-sitenetwork (non-U) must be questioned, but this style of access is all thatis needed for a user who works via an X.435 based broadbandstore-and-forward network with at least one on-site Node; the onlytheoretical limitation to such a mode of working is when livecommunication is necessary (e.g. a video link); however,store-and-forward has the very significant advantages of notinterrupting current work and of avoiding problems with timedifferentials for widely separated sites (e.g. for US and Europeancollaboration).

In order to enable management of the network a comprehensive signallingsystem must be provided. The signalling system is as described in GB2255257 and is described briefly below.

There are five kinds of equipment node in a network as described, thatis Switches, Public Network Ports Exchange Terminators (ET at theupstream end of an access link), Multiplexers and Network Terminators(NT at the downstream end of an access link). For Switches and PublicNetwork Ports a unique signalling address is allocated to each node butthis is not possible for peripheral nodes because the result would be ahalving of the number of Network Ports which may be served. Themechanism used for peripheral nodes, that is ETs, Multiplexers and NTs,is described below, with reference to FIG. 7.

Switches route on the destination address only. Signalling to peripheralnodes uses the normal destination address of the Network Port but one ofthree reserved addresses is carried in the VCI field, normally used tocarry the source address. The three addresses relate to the ET, theMultiplexer and the NT respectively. The displaced source address iscarried in the Information Field of the cell but since signalling isonly used intra-network, external equipment never sees this infringementof the "free-of-overhead" rule. All peripheral nodes are required totest all cells, on upstream and downstream intra-network ports, for thepresence of the relevant signalling address.

As well as access to the network manager, the signalling provides forNT-to-NT (i.e. User-to-User) signalling and ET-to-ET signalling for testpurposes and fault location.

Public Network Ports are allocated an address for signalling purposes.FIG. 8 shows how this address may be used in a reciprocal arrangementwith addresses allocated in both Source and Destination site networks.

A principal use of this mechanism is the establishment and change oftranslation tables in the two ends or the link, from an instance of theresource manager running in either network.

There is one area where the lack of full connectivity between sitesusing the present invention can be an embarrassment and that is for theubiquitous voice service; however, it its expected that the excellentservice provided by modern PABXs will continue to be used. One of theseveral options possible to avoid the inefficiency and cost of separateinter-site Synchronous Transfer Mode (STM) links for the PABX network isto adapt primary-rate multiplexes to ATM channels which may be carriedby the network. STM channels carried may use constant-bit-rate (CBR) orvariable-bit-rate (VBR) coding ith silence suppression and compression(e.g. using G.764/G.727 modified for ATM). Access into the STM publicnetwork will, however, not be possible by this means unless/until thepublic network operator provides a compatible service.

I claim:
 1. A telecommunications switching network comprising aplurality of sub-networks, each sub-network having a plurality of UserNetwork Ports wherein the Destination and Source addresses in acell-header carried by a cell switched by the network are each dividedinto a Port Address part and a Sub-network Address part andcharacterized in that the Source Sub-network part is implicit whilst thecell remains in the Source Sub-network and similarly the DestinationSub-network part is implicit from when the cell arrives in theDestination Sub-network, wherein the Source and the DestinationSub-networks are linked via a cross-connect switching device wherein theaddress formats are changed.
 2. A switching network as claimed in claim1, wherein the cell-header includes a format indicator to indicate theuse of a Source or Destination format.
 3. A switching network as claimedin claim 1, wherein the cell-header includes an alternative route bit toprovide alternative paths for a cell to a group of User Network Ports(UNPs).
 4. A switching network as claimed in claim 1, wherein thenetwork includes a public network, the User Network Interface formatincluding the space occupied by the Genetic Flow Control Field.